Solid state power combiner

ABSTRACT

An improved solid state transmitter (and elements therefor) adapted particularly well to pulsed operation at radio frequencies is disclosed. Such transmitter includes the combination of: A crystal-controlled oscillator producing a continuous wave output signal which, ultimately, determines the frequency of each transmitted pulse; a first oscillatory circuit, including a resonant cavity and at least one normally quiescent coaxial oscillator incorporating an IMPATT diode; a second oscillatory circuit, including a resonant cavity and a plurality of normally quiescent coaxial oscillators, each one of such oscillators incorporating an IMPATT diode; and an improved modulator for periodically actuating all of the IMPATT diodes in such a manner that a pulsed output of the first oscillatory circuit is produced which remains locked to the then existing continuous wave signal out of the crystal-controlled oscillator and the pulsed outputs of the coaxial oscillators in the second oscillatory circuit similarly are locked. 
     The improved elements disclosed, in addition to the improved modulator, include various configurations of coaxial oscillators which are easier to align than known oscillators of such type or which allow a greater number of such oscillators to be coupled to a resonant cavity than was possible according to the prior art. Finally, an improved probe and tuning arrangement for a resonant cavity is disclosed.

BACKGROUND OF THE INVENTION

This invention pertains generally to radio frequency oscillators andparticularly to oscillators of such type which are adapted to combinethe power out of a plurality of solid state devices.

It has been known for many years that radio frequency signals out ofeach one of a plurality of oscillatory circuits may, in effect, be addedin so-called "combiner" circuits to produce a single radio frequencysignal of high amplitude. With the development of solid state devices,such as IMPATT diodes, as the active elements in oscillatory circuits,the interest in combiner circuits has increased. The average and peakpower levels of these devices are high enough so that useful transmitterpower levels can be achieved by combining a practical number of devicesin a suitable combiner circuit. Exemplary combiner circuits using solidstate devices are shown in U.S. Pat. Nos. 3,628,171 (Kurokawa et al.)and 3,931,587 (Harp et al.).

Both of the just-cited patents show combiner circuits with a pluralityof IMPATT diodes operated as continuous wave (CW) oscillators, each oneof such diodes being in an oscillatory circuit which is coupled to acommon cavity. The requisite frequency and phase relationship betweenthe radio frequency oscillations of the different CW oscillators isdetermined in operation by the characteristics of the common cavity.

Although either of the combiner circuits shown by Kurokawa et al. andHarp et al. is satisfactory in its "steady state" condition, i.e. whenproducing CW oscillations, a somewhat different situation obtains wheneither is used to produce pulses of radio frequency energy. Pulseddesign can be optimized for simultaneously achieving stability ofoperation, combining efficiency and spectral purity.

Another problem with pulsed IMPATT diode oscillators, not addressed ineither of the cited patents, is that such devices require, for bestoperation, an electrical power supply which is effectivelycurrent-regulated in a particular manner. Specifically, the electricalpower supply must, if the spectral purity of each pulse is to bemaintained, be adapted to compensate for an increase in the temperatureof the junction of the IMPATT diode during the generation of each pulse.In addition, when pulsed operation is desired, the rise and fall timesof each pulse should be controllable to allow the spectrum of each radiofrequency pulse to be shaped as desired.

SUMMARY OF THE INVENTION

It has been suggested by Kurokawa et al. that a combiner circuit may bemade with a common rectangular cavity operating in a mode other than theTE_(01N) mode, where N is an integer corresponding to half the number ofcombined devices. The specific example given by Kurokawa et al. is acommon rectangular cavity operating in the TE_(02N) mode. Kurokawa etal. also suggest that conventional mode suppressors may be used when thecommon cavity is dimensioned to support the TE_(02N) (or higher) mode.The common cylindrical cavity shown by Harp et al. is operated in theTM₀₁₀ mode, although, presumably, mode suppressors could be incorporatedto allow higher modes, e.g. the TM₀₂₀ mode, to be supported. In anyevent, because of the fact that the longitudinal axes of the diodeoscillators and the cavity shown by Harp et al. are parallel to eachother, the maximum number of diode oscillators which may be coupled to acommon cylindrical cavity with a given circumference is determined bythe ratio of that dimension to the outside diameter of a diodeoscillator. Such a limitation on the maximum number of diode oscillatorsin turn places an unwanted upper limit on the amount of radio frequencyenergy which may be combined.

Therefore, it is a primary object of this invention to provide animproved "solid state" transmitter utilizing pulsed IMPATT diodes (orother such devices) whose power is combined in a combiner circuit.

Another object of this invention is to provide an improved combinercircuit for IMPATT diodes wherein the frequency at which such diodesoperate is determined by an "injection lock" technique whereby suchfrequency is controlled by a crystal oscillator.

Another object of this invention is to provide an improved combinercircuit for IMPATT diodes wherein the number of such diodes may be atleast double the number of IMPATT diodes arranged according to the priorart.

Still another object of this invention is to provide an improvedcombiner circuit for IMPATT diodes wherein such diodes are biased duringpulse operation in such a manner that the frequency of operation issubstantially unaffected by change in the temperature of the junctionsof such diodes.

The foregoing and other objects of this invention are generally attainedby providing, in a solid state transmitter, a combiner circuit using aplurality of pulsed diode oscillators coupled to a cylindrical cavity,the radio frequency energy in such cavity being injection locked duringeach pulse to the radio frequency energy out of a crystal controlledoscillator (which is operated continuously). The pulses of radiofrequency energy out of the pulsed diode oscillators are periodicallyproduced by applying direct current signals to an IMPATT diode in eachpulsed diode oscillator, such signals being derived from modulatorswhich are adapted appropriately to bias the IMPATT diodes. The inventionalso encompasses alternative combiner circuits wherein the number ofpulsed diode oscillator circuits may be maximized.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this invention, reference is nowmade to the following description of embodiments of this inventionillustrated in the accompanying drawings, wherein:

FIG. 1 is a block diagram, somewhat simplified, of a solid statetransmitter in accordance with the concepts of this invention;

FIG. 2 is a cross-sectional view of an exemplary one of the diodeoscillators used in the transmitter shown in FIG. 1;

FIG. 3 is a schematic diagram of an exemplary one of the modulators usedin the transmitter shown in FIG. 1;

FIG. 4 is a sketch, somewhat simplified, showing the physicalrelationship between the power combining and coaxial oscillator of FIG.1; and

FIGS. 5A, 5B and 5C are sketches showing how the diode oscillator shownin FIGS. 2 and 4 may be modified to increase the number of IMPATT diodesto be used in conjunction with an output cavity such as is shown in FIG.1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, it may be seen that a solid state transmitter11 according to this invention comprises two stages (not numbered), eachstage being associated generally with one of a pair of circulators 13,15 of conventional construction. The first, or low power stage,associated with the circulator 13, comprises a cavity 17 to which acoaxial oscillator 19 is magnetically coupled (as indicated by thebroken lines). The second, or high power output stage associated withthe circulator 15, comprises a cavity 21 to which each one of aplurality (here nine in number) of coaxial oscillators 23 ismagnetically coupled (as indicated by the broken lines). The coaxialoscillator 19 and the coaxial oscillators 23 (shown in detail in FIG. 2)are actuated by output signals from modulators 25 which, in turn, areeffective to apply biasing voltages from a D.C. power supply 27 to thecoaxial oscillator 19 and the coaxial oscillators 23 whenever actuatingsignals from a synchronizer 29 are applied.

Cavities 17 and 21 here are cylindrical cavities illustrated in moredetail hereinafter. Suffice it to say here that such cavities preferablyare dimensioned to support the TM₀₁₀ mode of oscillation at the radiofrequency to be propagated. Further, such cavities are proportioned tohave a optimum Q and their inner walls are coated with a highconductivity material and/or polished to reduce ohmic losses.

To complete the illustrated transmitter, a crystal controlled oscillator31 is connected as shown to the circulator 13 and an antenna assembly 33is connected as shown to the circulator 15. The crystal oscillator 31may be of any conventional construction to produce continuousoscillations at the frequencies within the locking bandwidths ofcavities 17 and 21. The antenna assembly 33 also may be of conventionalconstruction. However, it is preferred that the antenna assembly 33 be amonopulse array antenna with the output of the circulator 15 beingconnected through isolator means (not shown) to the sum port of thearithmetic unit (not shown) of such an array antenna. The differenceports of the arithmetic unit would then be connected to the differencechannels of a monopulse receiver (not shown) and the proper port of theisolating means would be connected to the sum channel of such receiver.

It wil now be recognized that, in operation, the output of the crystaloscillator 31 is continuously applied, through the circulator 13, to thecavity 17. Oscillations at the frequency of the crystal controlledoscillator 31 are, therefore, continuously induced in the cavity 17. Itfollows, then, that whenever the coaxial oscillator 19 is pulsed, theresulting pulsed oscillations are locked to those produced by crystalcontrolled oscillator 31. That is to say, the center line of the pulsedspectrum of coaxial oscillator 19 is locked to the frequency determinedby the crystal controlled oscillator 31. The output of the cavity 17 ispassed through the circulators 13, 15 to the cavity 21. Coaxialoscillators 23, in turn, are forced to resonate in phase with each otherat the resonant frequency of the cavity 21. The powers of the coaxialoscillators 23 are, therefore, combined in the cavity 21. When thepulsed oscillations from cavity 17 are present the high power spectrumfrom cavity 21 aligns itself with the spectrum of such oscillations. Thepulsed oscillations out of cavity 21 are then an approximate amplifiedreplica of the pulsed oscillations out of cavity 17.

It will be obvious to one of skill in the art that the resonantfrequencies of the cavities 17, 21 used in the solid state transmitter11 should, for best operation, be the same as the frequency of theoutput of the crystal controlled oscillator 31. Further, it will beobvious that any misalignment experienced in any practical case must beless than the locking bandwidths of the combinations of the cavity 17with the coaxial oscillator 19 and the cavity 21 with the coaxialoscillators 23. Locking bandwidth is inversely proportional to thelocking gain. For the typical gains (10-15 dB per stage) used in thesolid state transmitter 11 the bandwidth of the combination of thecavity 17 with the coaxial oscillator 19 is several tenths of a percentand the combination of the cavity 21 with the coaxial oscillators 23 isunder 3 percent.

It will be obvious to one of skill in the art that wide ambienttemperature variation and maximum temperature are two factors affectingtransmitter design. Upon turn-on, the IMPATT diode junction temperaturein each one of the coaxial oscillators 19, 23 rises above ambient by200° C in a fraction of a second. At junction temperatures above 220° Cthe IMPATT diode reliability is reduced by approximately one-half forevery 10° C increase in temperature. While the foregoing suggest that atemperature control arrangement of some type should be employed, it hasbeen found to be adequate simply to juxtapose cavities 17 and 21 and toprovide cooling coils of a conventional type (now shown in FIG. 1) for acoolant (such as water) to dissipate the heat generated at the junctionsof the IMPATT dioes in diodes coaxial oscillators 19, 23.

Referring now to FIG. 2, it may be seen that each one of the coaxialoscillators 19, 23 of FIG. 1 comprises an IMPATT diode 40 mounted in asection of coaxial line (not numbered). Each such line is herefabricated by forming a substantially cylindrical opening in a block(not numbered) of aluminum to make an outer conductor 42 with a centerconductor 44 supported therein in a manner to be described. The IMPATTdiode 40 is mounted between a heat sink 46 (which is supported by theouter conductor 42, as shown) and a metallic cup 48 slidably supportedon the center conductor 44. The IMPATT diode 40 preferably is maintainedin position by bonding with a conducting epoxy in a depression (notshown) in the metallic cup 48, and by soldering in an opening (notnumbered) in the heat sink 46.

An insulating sleeve 50 is disposed, as shown, to isolate the metalliccup 48 from the outer conductor 42 and the heat sink 46. The latter isheld in place by a threaded member 52. A spring 54 is disposed betweenthe metallic cup 48 and the center conductor 44. A first impedancetransformer (not numbered) here made up of the metallic cup 48, theinsulating sleeve 50 and a sleeve 55 (here made of beryllium copper), isdisposed as shown adjacent to the IMPATT diode 40. A second impedancetransformer (not numbered), here comprising a sleeve 56 of anyappropriate dielectric material bonded to the center conductor 44 andslidably disposed within the outer conductor 42, is also emplaced asshown. The purpose of the two impedance transformers will be discussedin more detail hereinafter. A termination load 58, here tapered andpositioned as shown against a conforming shoulder (not numbered) in theouter conductor 42, is slidably mounted on the center conductor 44. Thetermination load 58 is held in place against the conforming shoulder byan insulating cup 60, such cup being forced against the termination load58 by a threaded metallic member 62. The material of the terminationload 58 here is the material known as "ECCOSORB", (a trademark ofEmerson & Cuming, Incorporated, Microwave Products Division, Canton,Mass.). The threads on the threaded metallic member 62 are such as tomate with a corresponding threaded portion (not numbered) on the centerconductor 44. An insulating adjustment member 64 is placed on a formedend (not numbered) of the center conductor 44. Finally, a bias wire 66is supported in a feedthrough 68 (here any conveniently formedinsulating material) passing, as shown, through the outer conductor 42.The inner end of the bias wire 66 is connected in any convenient fashion(as by soldering) to the threaded metallic member 62. Finally, the outerconductor 42 is opened into a cavity 70.

It may be seen from the foregoing description that: (a) if the bias wire66 is connected to a source of electrical power (as one of themodulators 25 of FIG. 1), a voltage may be applied, through the threadedmetallic member 62, the center conductor 44, the spring 54 and themetallic cup 48 to one electrode of the IMPATT diode 40; and (b) thedistance between the sleeve 56 and the sleeve 55 may be changed byrotation of the insulating adjustment member 64.

The voltage applied in this case through the bias wire 66 ultimately tothe IMPATT diode 40 is derived from one of the modulators 25 (FIG. 1) ina manner to be described hereinafter. Suffice it to say here that thatelement is arranged to produce: (a) a pedestal voltage (meaning aconstant D.C. voltage) to bias the IMPATT diode 40 at a level somewhatbelow the level required for avalanche breakdown; and (b) at a selectedpulse repetition frequency, voltage pulses which are added to thepedestal voltage, thereby periodically to raise the level of the biasacross the IMPATT diode 40 to a level higher than the level at whichavalanche breakdown occurs. Typically, with an IMPATT diode such as thatdesignated "Part No. 5082-0710" (an X-band double drift IMPATT diode) bythe Hewlett Packard Company of Palo Alto, Calif., the pedestal voltageis in the order of 125 V. (D.C.) and the pulses in the order of 25 V.for 100 to 1,000 nanoseconds with a duty cycle of 30%. The current drawnfrom the modulator 25 (FIG. 1) is controlled to compensate for thefrequency change in the output of an IMPATT diode due to heating of itsjunction during each pulse. Thus, in the present case (where the levelof the radio frequency power out of a combiner circuit using ninecoaxial oscillators is to be in the order of 100 watts with a minimum offrequency chirp) the current in each pulse to each one of the coaxialoscillators is increased (as described in connection with FIG. 3) aseach voltage pulse is applied to the pedestal voltage. Finally, becausethe spectral purity of the radio frequency signal out of any pulsedoscillatory circuit is influenced by the shape of the leading andtrailing edges of the modulating signals in such a circuit, themodulators 25 (FIG. 1) are arranged to provide (as described inconnection with FIG. 3) pulses with leading and trailing edges whichhave adjustable rise and fall times.

The purpose of the first and the second impedance transformers is tomatch, at the desired frequency of operation, the relatively lowimpedance of the IMPATT diode 40 during each pulse to the relativelyhigh impedance of the cavity 70. The principles underlying the way inwhich such impedance matching may be effected are clearly shown in anarticle entitled "The Single Cavity Multiple Device Oscillator" byKaneyuki Kurokawa appearing in the IEEE Transactions on Microwave Theoryand Techniques, Volume MTT-19, No. 10, October 1971. The gist of thearticle just cited is that, assuming a "well-defined" admittance of eachIMPATT diode, the parameters of a single stage impedance transformer maybe defined to meet the necessary conditions for oscillations, whichinclude the following: (i) providing a load equal to the negative of thediode impedance at the desired operating frequency; (ii) presenting aproper phase angle between the load and diode impedance characteristics;and (iii) preventing oscillation at undesired frequencies. While theapproach taken in the just-cited article makes it possible to design aworking combiner circuit using a cavity to combine the outputs of aplurality of coaxial oscillators, such a design is difficult toimplement in practice.

The use of only a single stage impedance transformer (which inherentlyis a narrow band device) makes it difficult to simultaneously satisfyall of the criteria necessary for successful operation because of itslimited flexibility. The difficulty is compounded when a plurality ofIMPATT diodes is operated in 9 pulses mode in a plurality of coaxialoscillators. That is to say, when (as here) power from each one of aplurality of IMPATT diodes (each having different admittances varying ina nonlinear fashion with RF power and DC bias current) is to be combinedperiodically, it is almost impossible to avoid conditions which resultin unsatisfactory operation.

In order to provide additional tuning means which may be manipulated tosatisfy Kurokawa's criteria for oscillation and to allow for individualdiode tuning adjustments, a cascaded set of coaxial transformers,including one moveable transformer, is used in each diode line.

It will be remembered that the IMPATT diode 40 and the first impedancetransformer are parts of a unitary subassembly when emplaced in thecoaxial oscillator. It will also be remembered that the position of thesecond impedance transformer relative to the first transformer isadjustable. With such an arrangement, even though a deviation in theactual impedance of the IMPATT diode from its nominal value may induce acorresponding change in the impedance at the output of the firstimpedance transformer, compensation may be accomplished by adjusting theposition of the second impedance transformer relative to the firstimpedance transformer. Such adjustment, of course, is effective tochange the input impedance to the second impedance transformer, therebyfinally to cause the proper match between the output impedance of thattransformer and the input impedance of the cavity 70. It will be notedhere that the adjustment of the position of the second impedancetransformer relative to the first impedance transformer is accomplishedsimply by rotation of the adjustment member 64. This means that there isno opening required in the outer conductor 42 for access to the sleeve56. Such an opening would, of course, constitute a discontinuity toperturb the electrical field inside the coaxial oscillator.

Referring now to FIG. 3, it may be seen that an exemplary one of themodulators 25 (FIG. 1) comprises a voltage amplifier (VA) which, inresponse to control pulses from the synchronizer 29 (FIG. 1), drives acurrent source "A" and a current source "B". The voltage amplifier VA isa temperature compensated cascaded amplifier having two transistors, Q1and Q2, as the active elements therein (here, respectively, a type2N3866 and a type 2N2222A). The control pulses (which here typically arein the order of 800 nanoseconds in length at repetition frequenciesbetween 362 KHz and 435 KHz) are passed to the base of the transistor Q1through a coupling resistor R1A. The emitter of the transistor Q1 isconnected through a biasing resistor R1B and the parallel combination ofa capacitor C5 and a diode D5 to ground. The diode D5 here is a typeIN3611 diode to allow the desired temperature compensation to beeffected. The collector of the transistor Q1 is connected through acoupling resistor R2B to the emitter of the transistor Q2. The base ofthe transistor Q2 is connected to a diode D3 (here a type IN4148 diode)and a dropping resistor R3 to a 40 volt tap (not shown) in the D.C.power supply 27 (FIG. 1). The junction of the diode D3 and the resistorR3 is connected through zener diodes D1 and D2 to ground. The diodes D1and D2 here are type IN751A zener diodes. A bypass capacitor C1 isconnected across the diode D2. The junction of the diodes D1 and D2 isalso connected through a switching diode D4 to the junction between thecollector of the transistor Q1 and the coupling resistor R2B. Inaddition, a parallel combination of a resistor R2A and a capacitor C3 isconnected from the junction of the diode D3 and the base of thetransistor Q2 to ground. The collector of the transistor Q2 is connectedto the parallel combination of potentiometers R5A, R5B and resistor R6back to the 40 volt tap on the D.C. power supply 27 (FIG. 1). Thecollector of the transistor Q2 is also connected as shown to a capacitorC6 and the serial combination of a capacitor C7 and a potentiometer R7back to the 40 volt tap on the D.C. power supply 27 (FIG. 1). The tapson the potentiometers R5A and R5B are connected, respectively as shown,through resistors R8A and R8B to current source "A" and current source"B". Because such current sources are identical in construction only onewill be described. Thus, the second lead of the resistor R8A isconnected to the base of a transistor Q3A, here a type 2N3468transistor. The emitter of the transistor Q3A is connected through adropping resistor R(3EA) to the 40 volt tap in the D.C. power supply 27(FIG. 1) and directly to the base of a transistor Q4A. The latter hereis a type 2N5161 transistor. The emitter of the transistor Q4A isconnected through a dropping resistor R(4EA) to the 40 volt tap of D.C.power supply 27 (FIG. 1) and directly to the base of a transistor Q5A,here a type 2N5162 transistor. The emitter of the transistor Q5A isconnected through a dropping resistor R(5EA) to the 40 volt tap of theD.C. power supply 27 (FIG. 1) and to a zener diode D(A) poled as shown.The latter here is a type 1N4757 zener diode. The collectors of thetransistors Q3A, Q4A, Q5A and the second electrode of the zener diodeD(A) are connected together as shown to an input terminal of a pulsetransformer T(A) having a 1:1 turns ratio. The second input terminal ofthe pulse transformer T(A) is connected to ground. A serial combinationof a resistor R(FA) and a diode D(FA) is connected across the secondaryterminals of the pulse transformer T(A) along with oppositely polarizeddiodes D(PA) and D(LA). The just mentioned diodes all are type 1N4454diodes. The junction of the resistor R(FA) and the diode D(LA) isconnected to a tap (not shown) on the D.C. power supply 27 (FIG. 1)hereinafter referred to as the 120 volt tap. Finally, the output of thecurrent source "A" (labelled output "A") is taken at the junctionbetween the diodes D(PA) and D(LA).

Before an explanation of the just described circuit is undertaken itwill be appreciated that some simplification has been made. Inparticular, fuzes to protect the current source "A" and current source"B", arrangements for testing and parallel combinations of elements havebeen omitted.

The signal into the transistor Q1 is effective to change the currentflow through transistor Q2 in accordance with the setting of thepotentiometer R7 which, in turn, controls the time constant of thecombination of capacitor C7 and potentiometer R7. Such change in currentthrough the transistor Q2 then is reflected as a change in drive to thebase of the transistor Q3A by reason of the setting of the potentiometerR5A. The combination of the transistors Q3A, Q4A and Q5A is in effectsimilar in operation to a conventional Darlington circuit. The drive tothe base of the transistor Q3A determines the current level finallyattained by such circuit. The diodes D(PA) and D(LA) constitute atwo-way clamp whereby the voltage of output "A" is held at the level ofthe 120 volt tap in the D.C. power supply 27 (FIG. 1) at all timesexcept when a control pulse from the synchronizer 29 (FIG. 1) ispresent. The serial combination of the resistor R(FA) and diode D(FA) iseffective to quench any transient which may occur at the end of eachcontrol pulse from the synchronizer 29 (FIG. 1).

Referring now to FIG. 4, it will be noted that, because the coaxialoscillator 19 (FIG. 1) is here substantially the same as the coaxialoscillator shown in FIG. 2, a detailed description of the elementsmaking up the coaxial oscillator 19 is not necessary to an understandingof this invention. Further, it will be noted that the cavity 17 (FIG. 1)corresponds with the cavity 70.

With the foregoing in mind, it may be seen that the cavity 70 is hereformed by bolting together an upper body block 71 and a lower body block73 (the two such blocks preferably being flanged as shown and fabricatedfrom aluminum). The lower surface of the cavity 70 is that portion (notnumbered) of the lower body block 73 defined by a centrally locatedcounterbore (not numbered) shown in the upper body block 71. In theembodiment of the invention being described, the counterbore in theupper body block 71 is dimensioned to support the TM 010 mode at thefrequency of interest. A cylindrical hole (not numbered) parallel to thelongitudinal axes of the upper body block 71 and the lower body block 73is bored through such blocks, the axis of such cylindrical holeintersecting a circle, C. The radius of the circle, C, here is the sameas the radius of the counterbore in the upper body block 71. It will beobserved that the portions of the upper body block 71 and the lower bodyblock 73 surrounding the cylindrical hole through the upper body block71 and the lower body block 73 and the surfaces defining the cavity 70may be highly polished or plated in any convenient manner with amaterial, such as copper or silver, which is highly conductive to reduceohmic losses.

The lower body block 73 is extended as shown to provide room for anannular slot 75. A cover plate 75A is then positioned over the open sideof the annular slot 75 and secured in any convenient manner. An inletpipe 77 and an outlet pipe 79 are connected in any convenient fashion asshown to allow a coolant (such as water) to be fed from the pressureside of a pump (not shown) through the annular slot 75 to the suctionside of such pump. It will be appreciated by one of skill in the artthat the purpose of the illustrated cooling arrangement just describedis to remove heat generated within the IMPATT diode 40. In thisconnection it is here noted that the center conductor 44 and the heatsink 46 are in proximity to the IMPATT diode 40. For this reason, bothare made from oxygen-free high conductivity (OFHC) copper. As is known,OFHC copper is particularly well suited to resist thermal cracking.

A probe and tuning arrangement 80 here is positioned along thelongitudinal axis of the upper body block 71. Such arrangement hereincludes a probe section 82 within a tuning section 84, the two sectionsbeing mounted to be adjustable (either together or independently) in amanner now to be described.

The probe section 82 comprises a section of conventional coaxial line,i.e. a coaxial line having a center conductor 83, a dielectric spacer 85and a sheath 87, rotatably and slidably supported on the longitudinalaxis of the cavity 70. To effect such support of the probe section 80(and also to provide a similar type of support for the tuning section84) a probe adjusting member 89, here a metallic body machined to theshape shown, is threaded into a mating thread in a tuning adjustingmember 91. The latter is also a metallic body machined to the shapeshown, threaded into a mating thread in the upper body block 71 andjournalled in a bearing (not numbered) formed in the upper block 71. Itis noted here that a choke section 93 preferably is formed in the tuningadjusting member 91. A lock screw 95 threaded as shown into a matingthread in the tuning adjusting member 91 is positioned as desired eitherto lock the probe adjusting member 89 and the tuning adjusting member 91together or to allow those members to be moved independently of eachother.

It will be appreciated that the lower end of the center conductor 83must be electrically insulated from the tuning section 84. Suchinsulation is provided, as shown, by the lower part of the dielectricspacer 85 which is not removed when the lower part of the sheath 87 isremoved. It will also be appreciated that the coaxial line (notnumbered) in the probe adjusting member 89 must be connected to atransmission line (not shown) to allow, for example, a locking signal tobe injected into the cavity 70 and radio frequency energy to beextracted from the cavity 70. To accomplish this, the upper end of thecenter conductor 83 is exposed and a conventional coaxial connector 97is mounted on the probe adjusting member as shown. A conventional doublefemale adapter (not shown) may then be used to complete the requisiteconnection to a transmission line, terminated with a coaxial connectorsimilar to the coaxial connector 97.

It will now be apparent that the three adjustment points in thejust-described locking cavity and locking oscillator arrangement are allaccessible from the top. Therefore, in applications where space is at apremium, mounting problems are made less difficult to solve.

It will be appreciated that a plurality of coaxial oscillators similarto that illustrated in FIG. 4 could be disposed about the circumferenceof the circle, C. The total number of such oscillators is, of course,limited by the ratio of the largest diameter, d, of the coaxialoscillator to the circumference of the circle, C. With coaxialoscillators operating in X-band and a cavity supporting the TM₀₁₀ mode,it has been found that, as indicated in FIG. 1, up to 15 to 16 coaxialresonators may be positioned.

Referring now to FIG. 5A, it may be seen that a desired increase in thenumber of diode (or coaxial) oscillators disposed in a couplingrelationship about the periphery of a cylindrical cavity is hereeffected by changing the shape of each diode oscillator and modifyingthe way in which the cylindrical cavity is formed. Thus, in FIG. 5A itmay be seen that the center conductor of each one of a pair of diodeoscillators U,L is made up of two orthogonally disposed sections, e.g.center conductor 44AU and center conductor 44BU for diode oscillator Uand center conductor 44AL and center conductor 44BL for diode oscillatorL, joined in any convenient manner and centrally supported withinorthogonally disposed bores (not numbered) in either an upper body block71U or a lower body block 71L. The bores in which center conductors 44AUand 44AL are supported are parallel to the longitudinal axis of thecavity 70A, and preferably are centered at a common point on thecircumference of such cavity. The bores in which the center conductors44BU and 44BL are mounted are radial to the cavity 70A and are centered,as shown, to intersect, respectively, the bores for the centerconductors 44AU and 44AL. The cavity 70A is here formed by formingopposing counterbores in the upper block 71U and the lower block 71L. Itwill now be apparent that, with the radius of the cavity 70A the same asthe radius of the cavity 70 (FIG. 4) and the diameters of the diodeoscillators U, L the same as the diameter of the coaxial oscillatorshown in FIG. 4, twice the number of such oscillators may be disposed ina coupling relationship with the cavity 70A than with the cavity 70.

Because the center conductors of the diode oscillators being describedare bent, the way in which the elements making up each such oscillatorare mounted must, perforce, differ from the way in which correspondingelements in the coaxial oscillator described in connection with FIG. 4are mounted.

With the foregoing in mind, it will be seen that, in diode oscillator U,the center conductor 44BU is simply passed through an appropriatelysized hole (not numbered) in a termination load 60'. Such load is shapedas shown and cemented in the radial bore in the upper body block 71U.The free end of the center conductor 44BU then serves the same purposeas the bias wire 66 (FIG. 4).

The center conductor 44AU is passed through a sleeve 56' which issimilar in construction and purpose to the sleeve 56 (FIG. 4). In thiscase, however, a sliding fit is provided between the center conductor44AU and the sleeve 56'. Also, a slot (not numbered) is formed throughthe wall of the upper body block 71U adjacent to the sleeve 56'. It maybe seen, therefore, that the position of the sleeve 56' along the lengthof the center conductor 44AU may be adjusted without changing therelative positions of such conductor and the cavity 70A.

A depression (not shown) is formed in the free end of the centerconductor 44AU to accommodate one terminal of an IMPATT diode 40. Aconducting epoxy then may be used to provide a low resistance contactbetween the center conductor 44AU and the IMPATT diode 40. The secondelectrode of the IMPATT diode 40 is connected in the same way asillustrated in FIG. 2, i.e. such second electrode is soldered in anopening (not numbered) in a heat sink 46 which is held in place by athreaded member 52. An insulating sleeve 50' bonded to the heat sink 46and fitted around the center conductor 44AU completes the illustrateddiode oscillator U.

It will be recognized that the insulating sleeve 50', the sleeve 56' andthe termination load 60' serve to support the center conductors 44AU,44BU so that a bias voltage may be applied to the IMPATT diode 40. Inaddition, the insulating sleeve 50' and the sleeve 56' serve the samefunction as the two impedance transformers discussed in connection withFIG. 2. In the present case, however, the position of the sleeve 56' isadjusted through the opening in the upper body block 71U rather than byrotation of the adjustment member 64 (FIG. 2).

The diode oscillator L is made in the same way as the diode oscillatorU.

Although the bores in which the center conductors 44AU and 44AL aremounted are shown to be colinear, it will be evident that such anarrangement may be modified. That is, the only requirement here is thatthe bores be parallel to the longitudinal axis of the cavity 70A withtheir centerlines intersecting a circle so center conductor 44AU may beoffset from center conductor 44AL.

Before referring specifically to FIGS. 5A, 5B and 5C, it will be notedthat no cooling arrangement has been shown and that elements in thecoaxial oscillators which are the same as the elements illustrated inFIGS. 2 and 4 have been identified by the same numeral as in FIG. 2.

Referring now to FIGS. 5B and 5C, another modification is shown toincrease the number of diode oscillators which may be coupled to acylindrical cavity of a given size. In this case the number of diodeoscillators is limited by the ratio of the diameter, d, of each suchoscillator to the circumference, 2πR', of a circle greater than thecircumference, 2πR, of the cavity.

To effect the foregoing, advantage is taken of the well known fact thatthe dimensions (meaning the radii of the inner and outer conductors) ofa coaxial line may be changed without affecting the characteristicimpedance of such a line. That is, so long as the ratio between theinner radius of the outer conductor and the radius of the innerconductor are constant, the characteristic impedance of a coaxial lineis constant. Therefore, it is possible to proportion the radii of theinner and outer conductors of a coaxial line so that one portion of suchline (here the portion extending from a termination load past a cavity)is relatively small and a second portion (here the portion including anIMPATT diode) is sized to accommodate such a diode in an oscillatorycircuit.

With the foregoing in mind it may be seen in FIGS. 5B and 5C that anupper body block 71U' is machined to support a probe and tuningarrangement 80 (which may be the same as described hereinbefore) and aplurality of bores (not numbered) centered on a circle of radius R andparallel to the longitudinal axis of the upper body block 71U'. Suchbores, as indicated, are extended into a lower body block 71L'. Theradius of each bore is less than one-half the diameter, d, determined bythe diameter of an IMPATT diode 40.

The walls and bottom side of a cavity 70B are formed by a counterbore ofradius R in the lower body block 71L'. That block in turn is flared at aconvenient angle, say 45°, to its longitudinal axis. A plurality ofshaped bores (not numbered) is formed (as shown clearly in FIG. 5C) inthe flared portion of the lower body block 71L', each one of such shapedbores intersecting a corresponding one of the plurality of bores throughthe upper body block 71U' and a portion of the lower body block 71L'. Itwill now be evident that the surfaces of each shaped bore and itscorresponding bore make up the outer conductor of a coaxial line.

The outer portion of each shaped bore is machined to accommodate anIMPATT diode 40 and a first and a second impedance transformer in thesame way as described in connection with FIG. 5A.

The inner portion of each shaped bore is the same size as the bore withwhich it mates. Between the inner and the outer portion of each shapedbore there is a transition comprising a conical frustum.

A center conductor 44B having the cross-sectional shape shown in FIG. 5Cis supported in a coupling relationship with the cavity 70B within eachpair of bores and shaped bores. The upper end of the center conductor44B is supported by a termination load 60' and the lower end of thecenter conductor 44B is supported by the sleeve 58' and the insulatingsleeve 56'. The center conductor 44B is shaped so that, at any pointalong its length, the ratio of its radius to the radius of the shapedbore (or the bore) is constant.

It will now be apparent that, with a given diameter, d, as determined bythe IMPATT diode 40 and a cavity 70B of a given circumference, a greaternumber of diode oscillators may be coupled to the cavity 70B than wouldbe the case if the diode oscillators were configured as shown in FIGS. 2and 4.

Having described preferred embodiments of this invention, it will now beapparent to one of skill in the art that many changes may be madewithout departing from the inventive concepts. For example, althoughonly IMPATT diodes have been mentioned as the active elements, thedescribed oscillatory circuits are well adapted to use with other knowntypes of solid state diode oscillating devices. Further, while thecrystal controlled locking oscillator shown in FIG. 1 is here a CWdevice, it is obvious that such an oscillator may be replaced with apulse oscillator. Also, while the solid state transmitter of FIG. 1 isshown to combine the power from two oscillator stages, it is equallyobvious that three or more stages could be combined in like manner.Finally, the positions of the termination loads and IMPATT diodes shownin FIG. 5A could be interchanged. It is felt, therefore, that thisinvention should not be restricted to its disclosed embodiment, butrather should be limited only by the spirit and scope of the appendedclaims.

What is claimed is:
 1. In a transmitter of radio frequency energywherein pulses of radio frequency energy are, in response to controlsignals periodically produced by a synchronizer, generated by an IMPATTdiode, a modulator for converting each successive one of the controlsignals to an actuating signal for the IMPATT diode, such modulatorcomprising:(a) direct current biasing means for continuously applying adirect current signal to the IMPATT diode, the level of such signalbeing less than the avalanche breakdown level of such diode; (b) pulseforming means, responsive to each successive one of the control signals,to produce a corresponding voltage pulse having a level always greaterthan the difference between the level of the direct current signal tothe IMPATT diode and the avalanche breakdown level of such diode, thelevel of such voltage pulse increasing linearly with time; (c)transformer means, responsive to each corresponding voltage pulse out ofthe pulse forming means, for coupling to the IMPATT diode an analogouscurrent pulse as an actuating signal for such diode, the level of suchanalogous current pulse increasing substantially linearly with time; and(d) clamping means responsive at the end of each actuating signal tomaintain the level of the direct current signal applied to the IMPATTdiode at a constant value.